The present invention relates to an apparatus for cleaning and preparing grossly soiled products of plastic material, such as sheets, bands, sacks or the like for subsequent re-use of the plastic material, in which the plastic material after removing rough foreign objects therefrom is comminuted and fed into an extruder.
Such apparatus have become important in recent times, due to the increasing shortage and cost of raw materials as well as in consideration of the environment, to prevent such plastic products, after use of the same, to form part of an ever increasing amount of garbage which has to be dumped or burned.
During the production of, for instance, sheets or bands of plastic material, there will result a certain percentage of clean dry scrap which, separated in accordance with its composition, may be comminuted and without further treatment fed into an extruder. Such scrap constitutes about 10% of the material necessary for producing the aforementioned or like products. 90% of the material is transformed into products for instance sheets or bands which are mainly used in the packaging industry. Plastic material used for the packaging industry are usually discarded after a relatively short time and such discarded plastic material are, at least partly, considerably soiled.
Attempts have already been made to clean and prepare such soiled products of plastic material which have been used for the same purpose, for instance as sacks for fertilizers, which accumulate after use in considerable amounts, in order to remove from such products foreign particles clinging thereto as for instance fertilizer particles, humus particles, sand, etc.
For this purpose it is known to free the solid products of plastic material from foreign particles, to subsequently comminute the products in a cutting mill and to feed them thereafter into a special regranulating extruder.
This known process has the disadvantage that fine grained foreign particles will cling, even after the comminuting of the plastic products to the latter and be thus fed with the comminuted plastic material into the extruder. This will result in clogging the sieves of the extruder as well as in an increased wear of the various parts thereof, and the necessity to exchange the clogged extruder sieves, which evidently will reduce the output of the extruder. In addition, very fine grained foreign particles will not be retained by the sieves of the extruder and thus be included in the end product produced so that such end product will be of inferior quality.
Apparatus in which flat pieces of scrap of plastic material are moved around in a washing solution for separating foreign particles therefrom are known.
For instance an apparatus is known into which such scrap in form of rather large irregular pieces is fed and moved around in the washing solution by fast rotating members to be thereby torn into smaller pieces. Soluble foreign particles and unsoluble very fine grained foreign particles are thereby transmitted to the washing solution. The container of this known washing apparatus has a sieve basket in which comminuting and stirring parts of the apparatus rotate. The washing solution is discharged at the end of the washing process and the rotating elements tear, squeeze and rub the particulated plastic material further and to produce thereby heat by friction. The thus created heat will plasticize the comminuted plastic particles and evaporate the remaining washing solution. It is also possible to rinse the comminuted plastic particles with fresh water after the washing process and to evaporate the remaining water by the aforementioned heat created by friction.
This known device has considerable disadvantages. The sieve basket lets only soluble and very fine grained unsoluble foreign particles pass therethrough. Sand or even larger foreign particles are retained within the basket together with the comminuted plastic particles. Since the comminuted plastic particles are heated to such an extent to render the same into plastic condition, such sand particles or larger particles are welded together with the comminuted particles of plastic material and remain therefor in the cleaned material. This causes clogging of the sieves in the extruder in the subsequent extruding process and practice has shown that the sieves sometimes remain thus in proper working condition only for about 10 minutes.
It is further known that producing of heat by friction is very uneconomical since only that portion of the expended energy is transformed into heat which corresponds to the friction coefficient. As known, 540 kcal are necessary to evaporate one kilogram of water at atmospheric pressure while according to the mechanical heat equivalent 427 mkp mechanical energy is necessary for the production of one kcal. Therefore the output of cleaned comminuting particles of this known apparatus is small relative to the necessary large energy.
This known apparatus has further the disadvantage that it can be operated only in batches, that is intermittently, while requiring considerable manual labor.